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The KATANA Zirconia series allows flexible applications and the possibility of reproducing the variety of natural teeth in an efficient way. (Image: Kuraray Noritake Dental

Multilayered zirconia in different translucency levels

By Attila Kun
August 05, 2020

Kuraray Noritake Dental’s KATANA Zirconia ML was the first zirconia on the dental market with integrated shade gradation. First presented at the International Dental Show in 2013, this material has revolutionised the world of zirconia. Over the course of time, KATANA Zirconia UTML and STML and the change from ML to HTML have created a complete product portfolio. This article highlights the different characteristics of each of these varieties.

In daily clinical and laboratory routines, we quickly become aware of the diverse characteristics of natural teeth. Natural teeth impress with their individual optical properties, shape and texture. The challenge of imitating these aspects in ceramic restorations is a task that is handled in the dental laboratory with passion and motivation.

Accurate imitation of the natural tooth requires the appropriate framework material, for example KATANA Zirconia HTML, and an appropriate veneering ceramic or stain system, such as Noritake CERABIEN ZR, together with sensitivity and skill. Although modern materials lay down an important foundation, the implementation of the restoration is, to a large extent, an artistic skill. In order to select the appropriate material, depending on the indication, the dental technician should pay attention to facts and material science. Dental sensitivity and craftsmanship are also required for the aesthetic realisation of the prosthesis.

No matter whether it is for a monolithic restoration, thin-layer veneer or an individually layered restoration, zirconia offers various advantages as a restorative material, including good mechanical properties and a high level of biocompatibility. In recent years, zirconia has been further developed and optimised through material modifications, leading to new generations of zirconia. The resulting dental materials are remarkable for their translucency and outstanding aesthetic properties. For certain indications, monolithic restorations can be created in such way that the optical properties barely differ from those of a veneered restoration.

The KATANA Zirconia STML layered structure. (Image: Kuraray Noritake Dental)

Looking back at KATANA’s history

Kuraray Noritake Dental launched KATANA Zirconia ML in 2013. It was the first zirconia on the market with a polychromatic shade gradient and has revolutionised the market. “ML” stands for “multilayered”, and its chroma and saturation decrease from the cervical to the incisal layer. The market responded very well to the polychromatic discs; therefore, Kuraray Noritake Dental took the next step. In 2015, the KATANA family grew with the introduction of two new translucent materials: KATANA Zirconia STML (super-translucent) and UTML (ultra-translucent). Translucency studies have revealed the very high light transmission of KATANA Zirconia UTML (43%) and STML (38%). It should be noted that the strength of the material drops with rising translucency.

In 2019, KATANA ML became KATANA HTML. This change included a shade extension, from six to 14 shades, and an adaptation to the VITAPAN classical shade guide to ensure a simpler shade selection for the dental technician and better communication between technician and dentist. A few years ago, new zirconia stains (CERABIEN ZR FC Paste Stain, Kuraray Noritake Dental) were also launched on the market, thus perfecting the KATANA range of materials for highly aesthetic yet durable zirconia restorations.

The Japanese word “katana” denotes a special kind of traditional samurai sword. The katana sword combines expedient form with artistic design. Dental technicians aspire to achieve this in their work too. KATANA Zirconia offers the optimal foundation for this. The multilayered KATANA materials display a smooth progression of shade and brightness from the cervical to the incisal layer. Experience shows that a functional and aesthetic restoration can thus be implemented in an efficient way.

Application of polychromatic zirconia

Fully anatomical, partially anatomical or as a framework, the polychromatic KATANA materials can be used in a variety of ways. The integrated shade gradient displays gentle nuances of enamel, dentine and cervical shades and, in the case of STML, a translucency gradient. Depending on the indication, the zirconia blanks open up different ways of achieving an aesthetic restoration. Especially for complex repairs and anterior restorations, the set-up is an indispensable foundation, because using the correct material alone is no guarantee of success. Precise planning is called for in order to conceive an overall image. The surface structure, shape and contours are built up manually in wax, and after a double scan, the wax-up is reduced digitally. This creates a dentine structure or framework that can be milled out of the chosen zirconia option.

Differences between and indications for KATANA Zirconia HTML, UTML and STML. (Image: Kuraray Noritake Dental)

KATANA multilayered zirconia allows the framework to become a shade-bearing foundation. The CAD construction of the restoration is made in a reduced anatomical crown shape. A dentine core is then milled from zirconia. The ceramic veneer is reduced to a minimum. For a vivid result, the framework can also be characterised with internal staining. Shrinkage during baking is unlikely, and shade stability comes from the framework. In addition, the thin veneer layer ensures low shrinkage and lays a secure foundation for a high level of stability and a low risk of chipping. The result is a natural-looking restoration with warmth and translucency.

KATANA Zirconia HTML

KATANA Zirconia HTML has a high level of flexural strength and is typically indicated for crowns and bridges. It is available in numerous shades that cover different requirements in the dental laboratory. The material offers optimal optical properties for frameworks—as incident light is transmitted, the stump remains concealed. Its flexural strength is approximately 1,125 MPa.

The framework is designed after cutback. The challenge of ceramic veneers lies in the subtle and often differing shading of the neighbouring natural teeth. These characteristics can be reproduced using the internal stain technique. Shade depth and three-dimensionality are created when the layering is then covered with lustre compound. Lustre compounds are a special feature of the Kuraray Noritake ceramic system, and they envelop the actual layering like a fine cocoon.

(Image: Kuraray Noritake Dental)

KATANA Zirconia STML

Aesthetic restorations need light and translucency, which KATANA Zirconia STML offers. The manufacturer’s addition of yttrium oxide modifies the zirconia, and this leads to greater variance in particle size and increased translucency. KATANA Zirconia STML also has a polychromatic shade gradient from the cervical to the incisal layer. In addition to the shade intensity, its translucency also varies. Therefore, this material is ideal for frameworks in the anterior region (up to three units). The lower translucency in the cervical area is optimal for the shade-bearing framework foundation. The balanced combination of varying chroma and translucency allow for the optical properties of natural teeth to be imitated to the best effect. The flexural strength of KATANA Zirconia STML is 748 MPa.

KATANA Zirconia UTML

UTML offers the highest translucency of the KATANA family. The material’s optical properties come close to those of glass-ceramics, which extends its range of indications to include monolithic restorations in the aesthetic region, for example veneers. KATANA UTML has a lower chroma than does conventional zirconia. This is achieved through a consistently high degree of transparency, which brings out the intrinsic shade of the dentine. KATANA UTML is indicated for veneers, onlays and full-contour crowns, and its flexural strength is approximately 550–600 MPa.

Wafer-thin veneers (0.3 mm) of KATANA Zirconia UTML on the model. (Image: Kuraray Noritake Dental)

For the purpose of phantom work, we used KATANA UTML to produce full-contour veneers. The wafer-thin veneers were milled with a minimum thickness of 0.3 mm. Despite the thinness of the layer, there were no fractures or chipping at the edges. Individual characterisation was achieved through the staining technique. The milled veneers display beautiful transparency. In order to perfectly bring out the optical properties, an adhesive bonding cement such as PANAVIA V5 from Kuraray Noritake Dental can be used for such delicate restorations.

Light and shadow

“What is essential is invisible to the eye” is a phrase that can be applied to aesthetic restorations. In order to obtain a perfect ceramic restoration, the shape, contour and surface texture are important factors that need to be considered. Therefore—no matter whether monolithic or veneered—the appropriate preparation of micro- and macro-textures should never be overlooked. The application of gold powder, for example, has proved to be helpful, since even very fine structures become visible under gold powder. As with black and white images, the eye is not distracted by shade effects. After incorporating the textures and the final touches, manual polishing and adjustment of the shine are undertaken.

Conclusion

The KATANA Zirconia series allows flexible applications and the possibility of reproducing the variety of natural teeth in an efficient way. The materials differ in their translucency and mechanical properties.

To summarise:

  • KATANA Zirconia UTML is suitable for full-contour crowns in the anterior and posterior regions and veneers, inlays, onlays and single crowns in the posterior region.
  • KATANA Zirconia STML is ideal for crowns and small posterior bridges.
  • KATANA Zirconia HTML is a high-strength framework material for crowns and bridges.

With this selection of zirconia materials, dental technicians will be well equipped and prepared for everyday work and can thus devote themselves to precise dental work based on individual specifications.

About the author

Attila Kun
Hannker Dental
Ludwig-Gefe-Str. 28
49448 Hüde
Germany

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